Infrared spectral detection systems are useful in a variety of applications, especially for ground and space based persistent surveillance. Impediments to field deployment of hyper-spectral detection systems include the detector's overall size, weight, and energy usage, which are driven in large part by the requirements of the cooling system for the spectrometer and detector. Therefore, room temperature operation of hyper-spectral detection systems has long been sought to eliminate the vast cooling requirements of conventional hyper-spectral detection systems. However, intrinsic semiconductor detector noise from dark current and radiation from the spectrometer walls presents a significant limitation on system signal-to-noise performance.
One conventional solution to this problem for high-spectral resolution is the use of signal multiplexing, such as Fourier transform spectrometry. This approach detects the entire signal spectral band simultaneously, thereby increasing the competitive position of the signal current with respect to the noise sources. However, even signal multiplexing is overwhelmed by large background flux at elevated temperatures, and at high spectral resolution, this approach fails to achieve quantum noise limited (QNL) performance. QNL performance, where detection sensitivity is limited only by the quantum shot noise of the signal, is a widely used measure with which to determine the performance of signal detection in spectrometry applications.
Another approach uses thermal bolometer detectors for room temperature operation. However, Johnson-Nyquist noise imposes a requirement on these systems for broadband operation, with bandwidths in a range of about 10-90 cm−1. In addition, the thermal response times for bolometers impose limits on the operational speed of a spectrometer.
Another approach has been used to achieve QNL performance by heterodyne detection by using a single frequency laser source. In heterodyne detection, a known light source is combined with an incoming signal on a non-linear detector to produce beat frequencies that are amplified and detected. If the known light source generates more detector signal than the sources of noise, a QNL results. Heterodyne detection has historically depended on sufficiently bright light sources provided by narrow beam lasers. These heterodyne approaches rely on tuning the narrow band laser over the spectral region of interest. This tuning of the laser over the spectral band of interest can considerably reduce the efficiency in gathering spectral information over an extended spectral region, however. Conventional incandescent light sources, while potentially bright enough, are extremely inefficient since a substantial fraction of their output is outside the spectral range of interest.
Another approach illustrated in the prior art is to use an immersion condensing lens to reduce the size of the detector. Because dark current is proportional to detector size, this discriminates against background and dark current. A factor of 16 in focal area reduction has been achieved by VIGO for HgCdTe detectors in the long-wave region through the use of high index immersion lens technology. However this reduction is not enough to overcome dark currents sufficiently to allow QNL performance.
Therefore, all these approaches have failed to overcome the imposed limitations on size, weight, and energy usage imposed by cooling requirements. Even if QNL performance is achievable with hyper-spectral detection systems at elevated temperatures, the size, weight, and energy usage limitations placed on implementation of these devices makes their use impractical. Therefore, a hyper-spectral detection system which could reach QNL performance while achieving practical implementation regarding size, weight, and energy usage would be very beneficial, particularly in military and law enforcement efforts to deploy highly sensitive instruments capable of ground-based persistent surveillance and micro-power space surveillance.